After some googling, I determined that it's safe to power the (nominally 9V-powered) Meade ETX-60 telescope from 12V, so I had some use for the gel-cell battery I was using with the barndoor.
Here we can see the telescope, hand controller (with a Motorola MC68HC11 processor), 12V gel cell, and a digital multimeter which I bought for PHP 65 (about $1.50) some years ago.
Following a tip I saw on Weasner's Mighty ETX web site, I coiled the hand controller's cable around the power cable from the battery. This leads to neater cabling, and reduces the probability of tangling as the telescope slews around merrily.
I soldered a power cable to the gel cell's terminals, terminated on the other end with alligator clips. I just clip these to the 9V power terminals of the Meade. A disaster waiting to happen, if the clips get reversed. I think I'll put a diode in the Meade's battery compartment to prevent reverse-polarity connections.
Even while slewing on both axes, the gel cell comfortably powers everything, very minimal voltage drop. I think it's a 4.5-Ah unit; since the Meade draws at maximum 450mA, it should be good for ten hours at most.
My next plan is to built an adjustable latitude wedge, so I can polar-mount the ETX-60, then make a piggyback adapter so the Meade can carry my Canon DSLR. This will solve two problems: tracking and aiming. It will be trivial to command the Meade to point to a desired object, which it will then track while I take a picture with the piggyback-mounted DSLR.
ScopeTronix makes a camera piggyback adapter for the ETX-60 (see above). However it costs $50, so I'll make one of my own from scrap aluminum sheet or something.
I am just a bit concerned because people on the net are saying that the ETX-60 is not beefy enough to carry a DSLR. I think it can carry the Canon, so long as I only use a short and light lens like the EF 50mm f1.8. I doubt I will be able to use the 200mm f4 Pentax-screw mount that I bought for $10.